1. Field of the Invention
The present invention relates to adaptive optics. More particularly, it relates to the use of a vertical comb drive to actuate the deformable mirror in an adaptive optical system.
2. Description of Related Art
Atmospheric turbulence has frustrated astronomers ever since telescopes were invented. Atmospheric turbulence introduces aberrations on the wavefront of visible light, causing stars to twinkle and distant objects to shimmer. Atmospheric turbulence also limits the resolution of microscopes, endoscopes, and other optical imaging systems.
Adaptive optics systems can be used to measure and compensate for rapidly fluctuating wavefront distortions which blur the images of objects viewed through the turbulent atmosphere. While many adaptive optics systems are currently being used, they suffer from many shortcomings. Current systems are typically complex, expensive, unreliable and difficult to maintain, thus putting them out of reach of amateur astronomers or commercial users. Current adaptive optics systems are also very heavy and operate at relatively slow speed making them unsuitable for airborne applications where the fluctuating wavefront distortions move past the aperture at a much faster rate than stationary systems.
FIG. 1 illustrates a simple adaptive optics application. A micromachine deformable mirror 100, fabricated as an integrated circuit, is mounted onto a printed circuit board substrate 110. A wavefront sensing and correction microchip 120 is mounted on the substrate 110 next to the deformable mirror 100. The printed circuit board contains drive electronics 115 that connect the wavefront sensor chip 120 to the deformable mirror 100 chip. Incident light 130 is reflected off the micromachine deformable mirror 100. The reflected light 140 then passes through a beam splitter 150. A portion of the reflected light 140 is redirected onto the wavefront sensor 120 by the beam splitter 150.
The wavefront sensor 120 detects any aberration in the wavefront of the reflected light 140. Once it has detected the aberration, the wavefront sensor 120 then calculates the conjugate of the aberration. By applying the conjugate of the aberration to the incident light 130 as the light strikes the deformable mirror 100, the aberration is subtracted from the wavefront, and a corrected wave of light is reflected from the deformable mirror 100. The wavefront sensor 120 drives the deformable mirror 100 via the drive electronics 115 to correct for the detected aberration.
The primary obstacles to wider use of adaptive optics systems are cost, complexity, and reliability. Should reliable, low cost solid state adaptive optic systems become commercially available, many new applications can be envisioned. Laser communications is an example of such a market. Laser communication systems can support the signal bandwidth needed for High Definition Television without the need to run fiber optic cables between a remote mobile unit and a roving camera crew. Current laser communications have limited range due to atmospheric distortions that cause beam bending and scatter energy from the beam. An adaptive optic system capable of correcting for these effects can greatly extend range and improve performance.
A low cost adaptive optics system would find immediate applications in optical microscopes. Such a system could produce significantly improved imaging in difficult medical environments improving the performance of conventional and confocal microscopes. A low cost adaptive optics system has potential to enhance the operational performance of all clinical microscopes in use today.
A low cost adaptive optics system would also find applications in endoscopy. Here the adaptive optics system can be used for both imaging and transmitting the laser energy for endoscopic surgical procedures. There is considerable medical instrumentation and diagnostic equipment in the market place today. However, higher resolution imaging and more accurate placement of laser energy is needed to improve these instruments.
A low cost adaptive optic system could also find application in ophthalmology and optometry. In this application a small low power laser device is used to create an artificial guide star on the retina of the eye. The spot of light reflected off the retina forms a source to drive a wavefront sensor of an adaptive optic system. The kind of retinal camera that stands to benefit even more from adaptive optics is the confocal scanning laser ophthalmoscope (CSLO). Many clinical applications require the ability to optically section the retina in depth. Such sectioning can be achieved with confocal imaging in principle but current CSLOs do not tap the potential of confocal imaging because the retinal image quality is too poor. A CSLO equipped with adaptive optics could reap the full benefit of confocal imaging, improving the transverse resolution of current instruments by a factor of three and the axial resolution by a factor of ten.
Typical adaptive optics systems are composed of at least three core elements: (1) a wavefront sensor to detect optical aberrations, (2) electronic circuitry to compute a correction, and (3) a deformable mirror to apply the correction. The deformable mirror is a critical component of an adaptive optics system. It is used to apply the correction to the distorted wavefront. In current technology, the deformable mirror is also the most expensive component of the adaptive optical system. In order to realize a low cost adaptive optical system, a low cost deformable mirror must be developed. The current art presents three alternative technologies for deformable mirrors: liquid crystals, stacked piezoelectrics, and Micro-Electro-Mechanical Systems (xe2x80x9cMEMSxe2x80x9d).
Phase Modulating Liquid Crystal Display (LCD), devices offer low weight, low cost, and low power alternatives to large opto-mechanical devices. Also, cost effective bulk manufacturing methods currently exist for these devices. However, current LCD devices suffer from limited fill factor, limited bandwidth, and inadequate dynamic range. Bandwidth limitations will preclude the use of LCD""s for airborne or missile applications.
Stacked piezoelectrics (xe2x80x9cSPZTxe2x80x9d) utilize a new generation of piezoelectric technology that costs less and features the best advantages of actuators made from piezoelectric (xe2x80x9cPZTxe2x80x9d) or lead manganese niobate (xe2x80x9cPMxe2x80x9d) technologies. However, current SPZT devices suffer from high current operation, significant actuator nonuniformity, relatively high power dissipation, and moderate hysteresis effect. Moreover, these devices are relatively expensive when compared to liquid crystals or MEMS devices.
The recent advent of MEMS technology offers an alternative for the construction of cost effective mechanical mirror actuators. The technology is based upon the well established fabrication methods used to manufacture integrated circuits. Micromachining promises major improvements in overall performance and reduction of cost. The MEMS deformable mirrors have shown the lowest fabrication cost, lowest power consumption, lowest mass, lowest volume, elimination of hysteresis, elimination of polarization effects best power on, and lowest non-uniformity of any current deformable mirror technology. An additional advantage of this technology is that integrated electronic circuits can be fabricated directly on the same substrate as the micromirror. This allows for significant system simplification in that mirror drive electronics can be fabricated directly on the mirror substrate. MEMS technology offers light weight components, thus making it entirely suitable for ground based, airborne, and even hand held wavefront correction applications.
However, prior MEMS deformable mirror designs have been based on the electrostatic parallel plate capacitor actuators. This method of actuation has been successful but produces actuators with limited stroke, e.g., less than one micron, and a nonlinear voltage-versus-displacement curve. The nonlinear curve results from the 1/r dependence of the attractive force on gap distance. Recently, new MEMS actuator concepts have been demonstrated which use electrostatic comb drives to produce large stroke and linear voltage verses displacement curves. Unfortunately the current designs do not produce the vertical displacements necessary for use in an optical system.
It is therefore an object of the present invention to provide a low cost deformable mirror for use in an adaptive optics system which overcomes the problems noted above. It is further an object of the present invention to provide a deformable mirror with a high stroke and substantially linear voltage-versus-displacement curve.
These and other objects of the present invention may be realized by providing a deformable mirror by combining a vertical comb drive with a reflective surface. The deformable mirror may also include a spring for biasing the vertical comb drive to maintain the reflective surface in an original position absent application of a voltage to the vertical comb drive.
The vertical comb drive may include a first array of stationary elements and a second array of moving elements correspondingly interspersed with the first array. The reflective surface would be attached to the second array. A layer covering tops of elements of the second array may be provided. A spring may be used to suspend the first array relative to the second array, preferably via the layer. The stationary elements and the movable elements may be circular or planar. The layer may be attached directly to the reflective surface or with a post which is preferably in a center of the layer. Voltage may be applied to each stationary element of the first array individually or each moving element of the second array individually.
The vertical comb drive may include an array of vertical comb actuators. Each actuator of the array may be individually addressed to provide a voltage thereto. Springs for individually suspending each of the second arrays of each actuator in the array may be provided. Preferably, an anchor for supporting the springs is also provided.
The vertical comb drive may include a plurality of cavities and teeth interdigitally mounted with said cavities, the reflective surface being attached to the teeth. A top layer may be provided between the teeth and the reflective surface. A conductor for individually connecting each tooth of the teeth to a voltage source may also be provided.
These and other objects of the present invention may also be realized in accordance with a method of deforming a mirror including attaching the mirror to a vertical comb actuator and applying a voltage to the vertical comb actuator. The vertical comb drive may include an array of vertical comb actuators and the applying individually applies voltage to the vertical comb actuators.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.